1. Field of the Invention
The present invention relates to a technique of embedding information in a document and a technique of extracting embedded information.
2. Description of the Related Art
These days, it is very easy to copy, using a copying machine or the like, a document created by another person. Even a document which inhibits copying without permission, such as a document (to be simply referred to as a confidential document hereinafter) describing confidential matters, is highly likely to be easily copied. For example, when confidential information described in a confidential document is output as a printed material, the printed material is often copied. To prevent this, there have been proposed a variety of techniques for inhibiting arbitrary and intended copying operations or restricting them to a predetermined range.
Japanese Patent Laid-Open No. 7-231384 (reference 1) discloses a technique of generating a pattern image by image processing to obtain the same effects as those of copy forgery-inhibited paper. The copy forgery-inhibited paper has a special pattern image printed in advance. The pattern image is, for example, a hidden word of warning which is hardly seen by the human eye, but appears when copied by a copying machine. If a copying machine copies a document printed on copy forgery-inhibited paper, a word of warning such as “copy protected” appears conspicuously in the copy. Such a word of warning psychologically deters the user from illicit copying, and can distinguish a copy from the original. Reference 1 also discloses a technique of superposing the images of a warning word and background having undergone different dither processes at a specific common density, thereby compositing input image data and a pattern image capable of obtaining the same effect as that of copy forgery-inhibited paper.
Japanese Patent No. 3879552 (reference 2) discloses a technique of embedding mechanically readable copy protection information in a confidential document in advance, detecting it from an image read by a copying machine, and controlling the copying operation of the copying machine based on the detection result.
Japanese Patent Laid-Open No. 2004-221773 (reference 3) discloses a technique of creating an original by generating, with small independent dots, characters selected by the user from a document. When the original is copied, the characters disappear.
However, according to reference 1, when a copying machine copies a document, a word of warning appears in the background of the copy to psychologically deter the user from illicit copying. However, this is merely a deterrent effect, and cannot prevent illicit copying.
According to reference 2, it can be controlled to inhibit an image reading apparatus such as a copying machine from copying. However, the user can easily copy with popular image reading apparatuses which do not comply with the invention of reference 2. That is, if the user knows this fact and intentionally copies with a copying machine which does not comply with the invention, he can easily get a copy.
According to reference 3, when an original is copied, characters generated with small independent dots always disappear. Hence, even when the user legally copies, he cannot accurately reproduce the original, resulting in inconvenience. Every time the user creates an original, he needs to select characters to be generated with small independent dots. This puts a heavy burden on the user.